The main physical barrier limiting the effectiveness and velocity of transportation of liquids and in liquids lies in the fact that the driving systems have to overcome significant resistance accompanying the movement of vehicles or the movement of liquids transported through pipelines. The aero- and hydrodynamic resistance increases in proportion to the cube of relative velocity of the object and the fluid. One of the many ways proposed to reduce flow resistance of liquids (skin drag) is intentional modification of physicochemical and geometric properties of surfaces in contact with a flowing liquid. The relevant solutions to this problem described so far in the literature can be classified into two main categories, referred to as passive: or active ones.
Standard passive methods of lowering the resistance by limiting the development of turbulence involve, among other methods, modification of the geometry of objects around which a liquid flows. Most often flat and smooth surfaces, once considered optimal from the point of view of hydrodynanics, are covered with three-dimensional relieves. Known forms of surface three-dimensional arrangements and relieves have the form of parallel grooves (U.S. Pat. No. 5,171,623), microsteps (U.S. Pat. No. 5,133,519), depressions (U.S. Pat. No. 5,171,623) and scales (U.S. Pat. No. 5,114,099) with submilimeter to centimetre dimensions.
Also known are active systems of complex spatially arranged coatings equipped with a system of mobile, independent projections driven by micro servo-actuators, or tabs equipped with individual pressure sensors controlling each projection separately. Such systems are designed to actively dump turbulence or, to be more precise, to reduce it. Also known are designs with injection of liquid polymers or fine-grain suspensions, e.g. asbestos or polymer fibres into the inter-face, into the zone where the vessel's bottom and sides are in contact with water flowing around them.
In U.S. Pat. No. 5,445,095, a combination of the above-presented technologies has been proposed: injection of polymers onto a surface with parallel grooves; similar combinations of active and passive methods to reduce resistance are also shown in U.S. Pat. No. 4,932,612.
Also a design referred to as “sliding wall” was proposed involving the use of coatings stationary relative to the liquid and mobile relative to the flowing body.
The most recent and most effective among the known methods of reducing hydrodynamic resistance involve the reduction or almost complete elimination of immediate contact between the liquid and the object it flows around. It can be achieved by separating the two phases by a gaseous phase, for instance, by injecting compressed air into systems of straps, channels or shelves placed in the vessel's plating, by injecting a cloud of gas bubbles into the contact zone, or by employing the phenomenon of supercavitation. The injection of gas bubbles into the liquid-solid contact zone can be done through a porous, permeable surface or through a system of minute channels and nozzles, which in both cases require the use of gas distribution systems and also, in the case of submarine vessels, some storage facilities (patent application WO 8807956).
It was also proposed to force exhaust gases under the vessel's bottom or to employ the air entrained owing to viscous friction and later spontaneously sucked in by a system of appropriate nozzles (U.S. Pat. No. 5,545,063).
Some other radical solution reducing hydrodynamic resistance employs the so-called supercavitation a phenomenon involving the formation of a spindle-shaped cavity filled with water steam, moving along with the object. Such a cavity forms spontaneously around a body travelling through liquid environment at velocities greater than 50 metres per second and requires an appropriate, rounded bow. Such great velocities require very effective drive systems, e.g. a rocket engine.
A permanent, stable gaseous film (passive “lubricant”) can be much easier produced between a solid and liquid (in relative movement to each other) by making the solid's surface superhydrophobic. Superhydrphobicity means that a liquid has a contact angle between 160 and 180 degrees on the surface; in the case of nonwettability by other liquids, e.g. fats or mineral oils such phenomenon is, called lipophobicity. An example of material that is both hydro- and lipophobic is fluorine polymer Teflon®; a substance with properties opposite to those of diamond—a highly hydrophobic material that is wetted by fats—i.e. lipophylic. The phenomenon of superhydrophobicity is conditioned upon appropriate surface geometry. The contact angle of water measured relative to a completely flat surface of a solid never exceeds 140 degrees, even for most hydrophobic surfaces.
Also available on the market are numerous agents for making hydrophobic coatings, in the form of solutions to be applied onto surfaces, paints, self-adhesive foils, etc. The paints and coatings of this kind have a property permitting them to be self-cleaned from particles sticking to them—they also do not become frosted or iced. Due to the extremely low hydrophobicity of such artificial coatings a gaseous film is retained on the surface of a body placed in the bulk of a liquid, with no contact with its surface. This property permitting a gaseous film to remain under water has been employed to produce hydrophobic coatings designed to cover e.g., the bottom and the sides of a vessel or the interior of pipelines, thus reducing the viscous drag (U.S. Pat. No. 5,476,056).
Thus produced decrease in drag relates only to selected parameters of movement, and only to the vessel's flat bottom limited by side stripes. The greatest reduction occurs at low velocities: it must be accompanied by injection of additional portions of gas to make up the losses within the irregular gaseous film carried away by the moving, rough surface of the liquid flowing relative to the surface and also “scraped off” by the turbulence.
The hydrophobic surfaces employed so far do not have fully controlled geometry and are usually isotropic. They are formed, for instance, by wax crystallizing on a surface in the form of three-dimensional crystallites, or by spraying paints containing chemically hydrophobised silica grains.
U.S. Pat. No. 5,476,056 proposes such coatings of controlled geometry to be formed by e.g., by lithography, screen printing or electroforming. As a result of employing a chaotic or regular, yet isotropic geometry of the relief (patent application WO 0050232), such coatings can reduce the viscous flow resistance to a much lesser degree than it is theoretically possible when using a gaseous lubricant. The viscosity of air is hundreds times lower than that of water, and the viscosity of hydrogen is still lower, which is why the viscous drag of the fluid in contact with a gas film can be theoretically lowered by a factor of over a hundred as compared to a fully wetted immersed surface. Due to the isotropic, and usually chaotic and haphazard geometry of the coatings used so far, the surface in contact with the liquid's meniscus (the outermost, monomolecular meniscus i.e. liquid surface film, flexible due to surface tension) is covered with microcorrugations transverse to the direction of movement and reproduces, like a negative casting the spatial arrangement of the surface. Consequently, these roughness moving relative to the passing liquid “scratch” its deeper layers, more distant from the coating's surface. This causes the near-the-surface layer to be sheared and carried away with the movement of the rigid coating, the layer being carried away relative to the stationary bulk of the liquid, which leads to internal shearing and favours the creation of turbulence. Conversely, irregularities of the liquid's surface convex in the direction of the coating, carry away, scrape off and destroy the gaseous film.
The process of losing the hydrophobic lubricant is accelerated by “wind” waves, which appear on the liquid's surface—delicate after expansion of the gaseous cushion. Those “wind” waves develop due to fast jets of gas carried away with the gas layer. Irregularities of the gas film, thickened by injection of extra air, produce a similar effect. In the presented system with intense aeration through a single gap in the front part of the vessel plating, the coating can only be applied to the flat bottom rather than to the sides. Some parts of such chaotic coating are, due to accidental pattern defects, too irregular for the phenomenon of superhydrophobicity to occur and such thin liquid “bridges” connecting the liquid and the locally wetted defected surface not only increase the general drag, but can initiate intense turbulence and generate a cloud of bubbles, thus causing increased destruction of the gas film.
Anisotropic, superhydrophobic surfaces, linear in whole or in some sections, produced for experimental purposes by lithography on the surface of silicon crystals were also described (Bico J., Marzolin C., & Quere D., 1999, Pearl drops. Europhys. Lett. 47(2), pp. 220–226). The relieves under study contained only a simple system of parallel microgrooves and microribbs and the contact angle produced was slightly in excess of 130 degrees and depended strongly on the direction of the measurement. This type of surface arrangement has not been-proposed so far to reduce hydrodynamic resistance.
U.S. Pat. No. 5,054,412 discloses a solution which combines some of the known technologies described above: a system of macroscopic grooves parallel to the direction of flow, covered with hydrophobic coating. The grooves, constituting a structure that counteracts the development of turbulence, play the role of traps which retain the gaseous film and protect it against being washed out. The film is produced by injecting gas through a system of nozzles. In this solution macroscopic grooves of the same structural level were used
Another system in use is the technology of the underwater flocked coatings “Sealcoat” manufactured by the Creative Coating Corporation. “Sealcoat” consists of a layer of densely packed short, thin polymer threads, applied electrostatically directly onto the vessel's bottom or sides covered by appropriate resin. Such a coating, looking like velvet or seal, has high mechanical resistance and is also resistant, in spite of nontoxicity, to being inhabited by incrustating water organisms. Thus created coating is not planned to be hydrophobised. The fibrous “Sealcoat” coating is isotropic and chaotic. Fibrous textile hydrophobic materials are known as chaotic structures such as unwoven and woven fabrics. All known coating structures, designed mainly for the textile industry, are not adapted or designed to reduce hydraulic resistance.
The technological objective of this invention is to give to the coating such geometry that would make it hydrophobic, which means that the wetting angle relative to water or other liquids will be close to a straight angle that is its affinity to gaseous phase will exceed the affinity to liquid phase.
The superhydrophobic coating, optimized in terms of its ability to reduce viscous flow resistance, should be applied not only under the ship's bottom but also on its sides or inside a pipeline. The main parameters determining usefulness of such coating should be its mechanical and physicochemical durability, stability and uniform thickness of the gaseous film and the degree to which it reduces viscous resistance as compared to a fully wetted flat surface without any relief.